//文件夾路徑
string strFolderPath = Server.MapPath("~") + "\\excel\\";
DirectoryInfo dyInfo = new DirectoryInfo(strFolderPath);
//獲取文件夾下所有的文件
foreach (FileInfo feInfo in dyInfo.GetFiles())
{
  //判斷文件日期是否小于今天，是則刪除
  if (feInfo.CreationTime < DateTime.Today)
    feInfo.Delete();
}